Re: [PATCH v5 00/15] x86: Add support for Clang CFI

Peter Zijlstra
2021-10-27T12:47:07Z

On Wed, Oct 27, 2021 at 01:05:15PM +0100, Mark Rutland wrote:
> On Tue, Oct 26, 2021 at 10:16:22PM +0200, Peter Zijlstra wrote:
> > On Wed, Oct 13, 2021 at 11:16:43AM -0700, Sami Tolvanen wrote:
> > > This series adds support for Clang's Control-Flow Integrity (CFI)
> > > checking to x86_64. With CFI, the compiler injects a runtime
> > > check before each indirect function call to ensure the target is
> > > a valid function with the correct static type. This restricts
> > > possible call targets and makes it more difficult for an attacker
> > > to exploit bugs that allow the modification of stored function
> > > pointers. For more details, see:
> > >
> > > https://clang.llvm.org/docs/ControlFlowIntegrity.html
> >
> > So, if I understand this right, the compiler emits, for every function
> > two things: 1) the actual funcion and 2) a jump-table entry.
> >
> > Then, every time the address of a function is taken, 2) is given instead
> > of the expected 1), right?
>
> Yes, and we had to bodge around this with function_nocfi() to get the
> actual function address.

The patch set under consideration seems to have forgotten to provide one
for x86 :/

> Really there should be a compiler intrinsic or attribute for this, given
> the compiler has all the releveant information available. On arm64 we
> had to us inine asm to generate the addres...

Agreed, this *really* shouldn't be an arch asm hack trying to undo
something the compiler did.